In Patent Document 1, there is described a technology for forming a clear image with high light use efficiency in an image forming device including a phase-modulation type spatial light modulator. The device described in this document generates a Fourier optical image of a phase modulation optical image output from the spatial light modulator, and provides a predetermined phase shift for only the zero-order light component of the Fourier optical image, and takes an optical image which is obtained by performing inverse Fourier transformation of the phase-shifted optical image with an imaging device. This device compares the image obtained in this way and a target image input to the spatial light modulator, and carries out feedback control of a light source, the spatial light modulator, and the like, based on a difference between the images.